The subject invention relates to optical metrology devices which include a movable stage for rastering a wafer with respect to a measuring probe beam. More specifically, the invention relates to a stage which includes a system for mounting a reference chip within the footprint of the wafer thereby reducing the amount of stage travel necessary to measure both the wafer and the reference chip.
Optical metrology instruments require periodic monitoring and calibration. The output intensity of the light sources, the nature and extent of solarization of the optical components, chemical contamination of the optical surfaces and the alignment of the system optics can all vary with system operating time. An instrument""s performance must be regularly monitored to verify that the system continues to meet operational specifications and that measurements are performed with the required precision and accuracy. Frequently, this is accomplished with the aid of a reference sample. A reference sample is a well-characterized specimen with known and temporally stable optical properties. Any variation in the measurement of the reference sample optical response is indicative of a variation in performance of the instrument. It is the periodic measurement of the reference sample that indicates performance problems and the requirement for maintenance or re-calibration.
Optical wafer metrology systems are characteristically configured with the wafer surface approximately coincident with the focal plane of the optical system. The focal plane is flat and perpendicular to the plane of incidence of the probe beam (typically defined as the x-y plane). The vertical or z position of the wafer should coincide with the focus position of the probing beam.
High-resolution xe2x80x9csmall spot-sizexe2x80x9d optical wafer metrology tools illuminate a small portion of the wafer surface at the focal position and monitor the change in one or more properties of the reflected light caused by the interaction with the sample surface. Characteristically, measurements are made sequentially as a translating wafer stage moves the wafer surface xe2x80x9cthroughxe2x80x9d the illuminated region. Conventional wafer xe2x80x9cmasteringxe2x80x9d or translation protocols include both bi-linear, x-y translation and single-axis translation in combination with z-axis rotation. The stage system can also include z-axis movement for raising and lowering the wafer surface to achieve focus.
In the prior art it has been desirable to place the reference sample in the focal plane. If the reference surface is physically located within the same plane as the wafer surface, no substantial refocusing of the optical system is required during measurement of the standard sample. For systems employing x-y translation stages the reference sample is typically attached to the wafer chuck at the corner of the stage where it does not interfere with wafer measurements. For systems employing z-axis rotation stages, restrictions posed by rotation symmetry, the location of auxiliary metrology instrumentation and the location and design of the wafer handling equipment make locating the reference sample more difficult. Even when a suitable location can be identified this often requires a more expensive, long-travel stage to be used so that the reference sample can be moved to the measurement position. These factors increase both the complexity of the instrument and its cost and size.
Accordingly it would be desirable to locate the reference sample on the wafer chuck within the wafer footprint. This offers two important advantages. First, the stage-travel requirements are determined solely by the wafer dimensions. Therefore minimum form-factor wafer-translation systems can be employed. Second, a major limitation of the prior-art approach is eliminated permitting the use of compact wafer-translation systems having rotary stages. In particular, the prior approach of placing a reference chip outside a circular chuck and connected to the chuck cannot be implemented in a rotational system where an external pin lifter mechanism is used to raise wafer. As can be appreciated, if the reference chip extended beyond the circumference of the chuck, it would prevent the chuck from rotating since it would intersect with the pins of the wafer lifter. Placing the reference sample within the footprint of the chuck allows an external pin lifter to be used with a rotational chuck.
The subject invention relates to an apparatus for holding and translating a wafer in an optical wafer metrology tool. The apparatus incorporates a wafer-chuck that is attached to and combined with a wafer translation system. The apparatus further includes a holder for a reference specimen. The holder is installed within the body of the wafer-chuck within the area of the chuck used for wafer clamping. The holder is movable between a retracted position where the reference sample is below the chuck surface, and an extended position where the reference sample is substantially coincident with the wafer position. During wafer metrology the holder is maintained in the retracted position. Measurement of the reference sample is made with the wafer removed and the holder maintained in the extended position.
Locating the reference specimen within the area of the chuck used for wafer clamping enables an economy of design. The required range of stage-travel is set by the wafer dimensions. This admits the use of extremely short-travel translation stages to access large wafer areas. For example, the entire surface of a 300 mm diameter wafer can be accessed using a single 270 degree rotary-stage in combination with two xc2x175 mm linear-travel stages, e.g. xc2x175 mm of travel in the x direction and xc2x175 mm in the y direction. The economy of motion enables increased accuracy of wafer positioning and increased wafer-throughput at reduced cost.